In vitro fatigue crack analysis of the Lubinus SPII cemented hip stem

Fatigue damage of cement mantle of cemented hip replacements has been extensively studied. Different conclusions concerning the micro-cracking mechanism has been reported. An in vitro study on commercial cemented synthetic femurs was conducted to determine if micro-cracking formation of the cement mantle would preferentially be at the cement–stem interface or at the bone–cement interface. A stair climbing fatigue loading cycle was applied during one million cycles at 2 Hz on commercial stems. Three hip replacements with the Lubinus SPII cemented stem were studied. After the experiments each femur was sectioned at eleven sections, with an average of 5 and 10 mm of offset between each one of them and analysed with dye liquid penetrates.The results (micro-cracking and debonding) were influence by the geometries of the reamer and the stem and by technical replacement procedure. The micro-cracking formation initiated at the stem–cement interface towards the cortical bone of the femur. For a cement mantle thickness of more than 5 mm, cracking initiated at the cement–bone interface and depended on the geometry of the reamer. At the medial side of the femur more significant cracks and accumulated damage was observed at the stem–cement interface. Correlation between the formation of cracks and minimum cement thickness was determined. A critical cement mantle thickness was observed for 0.5 mm, where a relevant sign of damage was assessed. The analysis of crack-density and accumulated damage showed that fatigue induced cement damage and depended on the axial position of the stem and increase proximally.